Mexico City, Feb 1 (AP) Rescuers searched for survivors today and authorities promised a thorough investigation after an office building blast killed 32 people and injured 121 at the headquarters of Mexico's state-owned oil company, Petroleos Mexicanos. The cause of the basement explosion in an administrative building next to the iconic, 51-story Pemex tower in Mexico City remained a mystery, with President Enrique Pena Nieto urging people not to speculate. Theories ranged from an electrical fire to an air conditioning problem to a possible attack. "We have no conclusive report on the reason," Pena Nieto told reporters. "We will work to get to the bottom of the investigation to find out, first, what happened ... and if there are people responsible in this case, that we apply the full weight of the law against them." Pemex Director Emilio Lozoya said Friday that the death toll had risen to 32, with 121 people were treated for injuries after Thursday's blast. He said that rescue efforts continue and people still could be trapped. More than 500 firefighters, soldiers and rescue workers dug through chunks of concrete with dogs, trucks and a Pemex crane. Interior Minister Miguel Osorio Chong said it was uncertain if any of the roughly 10,000 people who work in the five-building headquarters were still trapped, but that the search would continue. The explosion occurred at about 3:45 pm, just as the administrative shift was about to end. It hit the basement and first two floors, which rescuers said all collapsed onto each other. "There is a lot of risk," rescuer German Vazquez Garcia said of working on the site. Pemex first said it had evacuated the tower and 14-story administrative building because of a problem with the electrical system. The company later tweeted that the Attorney General's Office was investigating the explosion. AP DDC